Casting process using borax-silica slag



United States Patent 3,225,399 CASTING PROCESS USING BGRAX-SILICA SLAGGeorge W. Belcher, Norwallr, Conn, assignor to Union CarbideCorporation, a corporation of New York I No Drawing. Filed Nov. 15,1961, Ser. No. 152,610 5 Claims. (Cl. 22-200) The present inventionrelates generally to a casting process and, more particularly, to aprocess for casting copper and copper-containing alloys.

Heretofore, the casting of copper and copper-containing alloys hasproduced a variety of defects in the cast products. Examples of suchdefects are porosity from liberaf tion of gases, refractory or drossinclusions, and surface defects such as cold shuts and bleeders. As aresult of such defects, a relatively large number of permanent moldcastings of copper or co-ppercontaining alloys must be rejected. In thecase of ingots, it is often necessary to remove defects by localchipping or grinding or even to remove the entire ingot surface bycutting or machining.

A number of casting processes have been previously proposed for avoidingthe formation of the aforedescribed defects in various metals by forminga shell of casting slag on the inner walls of the casting mold eitherprior to or simultaneously with the pouring of the molten metal. Thecasting slag has a freezing point below the freezing point of the metalbeing cast so that the metal freezes before the inner surface of theshell of casting slag. As a result, the metal freezes in contact with aliquid surface rather than the sol-id mold wall, and a smooth Surface isproduced on the cast product. Typical exam les of such casting processesare described in detail in U.S. Patent No. 2,631,344 to Kennedy and US.Patent No. 2,493,394 to Dunn et al. When the shell of casting slag ispre-formed over the entire inner surface of the mold, splashings fromthe molten metal being poured melt the inner surface of the shell at thepoint of impingement and fall back into the pool of molten metal ratherthan clinging to the mold wall and forming scabs and slivers on theingot surface. Moreover, when the molten metal is poured through a poolof the molten casting slag, the stripping action of the casting slag mayeffect further refining or purification of the molten metal. However,most such casting processes heretofore proposed have employed castingslags which are of little or no value in casting copper orcopper-containing alloys.

It is, therefore, the main object of the present invention to provide aprocess for casting copper or coopercontaining alloys into bodiessubstantially free of defects.

It is another object of the invention to provide such a process whereina shell of casting slag is formed on the inner walls of the castingform.

It is a further object of the invention to provide such a processwherein the copper or coppercontaining alloy is cast through a layer ofmolten casting slag.

Other aims and advantages of the invention will be apparent from thefollowing description and appended claims.

In accordance with the present invention, there is provided a processfor casting copper or copper-containing alloys comprising preparing aborax-silica casting slag having a freezing point below the freezingpoint of the material to be cast; forming a molten pool of the castingslag on the bottom of a casting form; forming a shell of the castingslag on the inner walls of the casting form; and pouring a molten massof the material to be cast into the casting form and through the pool ofcasting slag at a rate such that only the inner surface of the shell ismelted.

The casting slag employed in the inventive process is a mixture of boraxand silica having a freezing point below the freezing point of copper orthe particular copper-containing alloy to be cast. As used herein, theterm freez- 3,225,399 Patented Dec. 28, 1965 ing point refers to thetemperature at which any portion of the molten material is precipitatedor converted to the solid state. The amount of silica in thebo-rax-silica slag should be sufiiciently great to produce a glass-likesilicate shell which readily adheres to the walls of the casting mold,and it is generally preferred to have a silica content of at least 20percent by weight for this purpose. Also, the proportions of borax andsilica in the slag should be such that the freezing point of the slag isbelow the freezing point of the particular material to be cast. Sincethe eutectic composition has the lowest melting point in a given system,the preferred casting slag in the present process is usually theeutectic composition of the boraxsilica system, i.e., a slag of 72percent by weight RX and 28 percent by weight silica. This slag has amelting point of 1526 E, which is below the melting point of copper andmost copper-containing alloys. Of course, the composition of the slagcan be varied from the eutectic composition as long as the variationdoes not increase the freezing point of the slag above the freezingpoint of the material to be cast. Also, the slag may contain minorquantities of impurities which are substantially inert to the materialto be cast and do not substantially increase the freezing point of theslag.

The shell of casting slag can be formed on the inner walls of thecasting mold in various ways. For example, the molten metal may bepoured into a mold partially filled with the molten slag so that a layerof molten slag rises on top of the pool of molten metal and forms ashell on successive zones of the mold wall. Alternatively, the entireinner surface of the mold may be covered with a shell of casting slagprior to the pouring of the metal by completely filling the mold withmolten casting slag and allowing it to stand until a shell of thedesired thickness has solidified on the mold walls. A complete shell mayalso be formed prior to the pouring operation by blowing the castingslag in powdered form through a flame torch and onto the mold wall. Aflame torch suitable for such use is described in detail in U.S. PatentNo. 2,735,796.

The thickness of the shell of casting slag deposited on the mold wallsshould be sufficient to prevent penetration thereof by the molten metalbeing cast. Since a portion of the shell is melted by the introductionof the molten meta-l, the shell should always have internal dimensionssmaller than the external dimensions desired in the final ingot or castproduct. The slag which is melted from the shell during the pouringoperation rides on the rising pool of metal, thereby isolating the metalfrom the surrounding atmosphere, and allows the metal to solidifywithout any outside contamination. It is usually preferred to have aslightlv greater shell thickness at the bottom of the mold to providefor the initial impact of the stream of molten metal. Shells ofexcessive thickness should be avoided, since such shells do not havesufficient body to Withstand the heat shock from the molten metalwithout :spalling. Also, excessive shell thicknesses result in smallingots and may interfere with the removal of the ingot from the mold.The preferred shell thickness is less than about inch.

A pool of molten casting slag should be present in the bottom of themold when the molten metal is poured. The stream of molten metal passesthrough the molten casting slag, and the horax in the slag scavengesdross inclusions and gases from the metal. Thus. the inventive processeliminates both internal and surface defects in the cast product. Sincethe molten slag rides on top of the rising pool of meta-l, the incomingmetal continuously passes through a layer of molten slag. The de th ofthe molten slag should be sufiicient to effect the desired degree ofwashing or refin ng in the molten metal. Where the slag shell is formedby freezing from the rising layer of slag. the initial pool of slagshould be large enough to provide a layer of slag even when the surfacelevel of the molten metal reaches the top of the mold. When the slagshell is formed over the entire inner surface of the mold, additionalslag will be added to the rising layer by partial melting of the shell.

The inner walls of the shell of casting slag, i.e., the walls whichcontact the molten metal, should be at a temperature above thetemperature of the outer walls of the shell, i.e., the walls whichcontact the casting form. Preferably, the inner walls of the shellshould be at a temperature approaching the melting point of the castingslag so that metal splashing occurring during the pouring operationliquefy the surface of the shell at the point of impingement and are notretained on the inner surface of the shell. Since the melting of theinner surface of the shell causes the splashings to fall back into themolten metal being cast, they do not form scabs and slivers on the ingotsurface. It is also important that the outer walls of the shell be at atemperature sufiiciently low to freeze the molten metal so that themolten metal solidifies before coming into contact with the castingform. Since the temperature of the air surrounding the casting mold isnormally considerably below the temperature of the molten metal beingpoured, the desired temperature gradient in the shell is usuallyachieved without the use of cooling or heating devices.

In an example of the inventive process, 1000 grams of a borax-silicacasting slag consisting of 72 percent by weight borax and 28 percent byweight silica was poured into a casting mold and allowed to stand until.a slag shell was formed around the inner surface of the bottom of themold. A molten alloy containing 85 percent copper, 5 percent tin, 5percent zinc, and 5 percent lead, by weight, was then poured into themold and through the pool of molten casting slag at a rise rate of 58inches per minute. The temperature of the casting slag was 1300 C.,while the temperature of the molten alloy was 1200 C. After the ingothad solidified, it was removed from the mold and the slag enveloperemoved therefrom. The ingot was 3 inches square and 16 inches long,weighed 22 pounds, and was substantially free of 'both surface andinternal defects.

In a second example of the inventive process, 40 pounds of aboraX-sil-ica casting slag consisting of 72 percent by weight borax and28 percent by Weight silica was poured into a cylindrical, water-cooled,copper casting mold and allowed to stand until a slag shell was formedaround the inner surface of the bottom of the mold. A molten alloycontaining 97 percent by weight copper and 3 percent by weight siliconwas then poured into the mold and through the pool of molten castingslag at a rise rate of inches per minute. The temperature of the castingslag was 2400 F., while the temperature of the molten alloy was 2200 F.After the ingot had solidified, it was removed from the mold and theslag envelope removed therefrom. The ingot was 7.25 inches in diameterand 100 inches long, weighed 11 pounds, and was substantially free ofboth surface and internal defects.

While various specific embodiments of the present invention have beenillustrated and described herein, it is not intended to limit theinvention to any of the details herein shown, but only as set forth inthe appended claims.

What is claimed is:

1. A process for casting a material selected from the group consistingof copper and copper-containing alloys comprising preparing aborax-silica casting slag consisting of between about 20 and about 30percent by weight silica and between about 80 and about 70 percent byweight borax, and up to an insubstantial amount of impurities which areinert to the material to be cast and which do not substantially increasethe freezing point of the bo-raX-silica slag, said slag having afreezing point below the freezing point of the material to be cast;forming a molten pool of said casting slag on the bottom of a castingform; forming a shell of said casting slag on the inner walls of saidcasting form; and pouring a molten mass of the material to be cast intosaid casting form and through said pool of casting slag at a rate suchthat only the inner surface of said shell is melted.

2. A process for casting a material selected from the group consistingof copper and copper-containing alloys comprising preparing aborax-silica casting slag containing about 72 percent by weight boraXand about 28 percent by weight silica; forming a molten pool of saidcasting slag on the bottom of a casting form; forming a shell of saidcasting slag on the inner walls of said casting form; and pouring amolten mass of the material to 'be cast into said casting form andthrough said pool of casting slag at a rate such that only the innersurface of said shell is melted.

3. A process for casting a material selected from the group consistingof copper and copper-containing alloys comprising preparing a bath ofmolten borax-silica casting slag consist-ing of between about 20 andabout 30 percent by weight silica, between about 80 and about percent byweight borax, and up to an insubstantial amount of impurities which areinert to the material to be cast and which do not substantially increasethe freezing point of the boraxasilica slag, said slag having a freezingpoint below the freezing point of the material to be cast; intimatelycontacting the inner walls of a casting form with said molten castingslag; chilling said molten casting slag in contact with said inner wallsof said casting form to produce a shell of casting slag reinforced bysaid casting form; maintaining the inner walls of said shell at atemperature in excess of the outer walls thereof; and pouring a moltenmass of the material to be cast through a molten layer of said castingslag and into said shell at a rate such that only the inner surface ofsaid shell is melted.

4. The process of claim 3 wherein said borax-silica casting slagcontains about 72 percent by weight borax and about 28 percent by weightsilica.

5. A process for casting a material selected from the group consistingof copper and copper-containing alloys comprising forming in a castingmold a 'body of molten boraX-silica casting slag consisting of betweenabout 20 and about 30 percent by weight silica, between about 80 andabout 70 percent by Weight 'horax, and up to an insubstantial amount ofimpurities which are inert to the material to be cast and which do notsubstantially increase the freezing point of the boraX-silica slag, saidslag having a freezing point below the freezing point of the material tobe cast; allowing said molten casting slag to stand in said mold until athin shell of frozen casting slag is formed on the inner surface of thecasting mold; and pouring a molten mass of the material to be cast intosaid casting mold and through the body of molten casting slag therein sothat said molten casting slag rises on the pool of molten material beingcast at a rate sufiicient to form a thin shell of frozen casting slag onsuccessive zones of the inner surface of said mold.

References Cited by the Examiner UNITED STATES PATENTS 1,955,726 4/1934Archer et a1 75-76 2,493,394 l/19SO Dunn et al. 22-200 X 2,535,53612/1950 Hagmaier.

2,631,344 3/1953 Kennedy 2220O MARCUS U. LYONS, Primary Examiner.

DAVID L. RECK, MICHAEL V. BRINDISI, WILLIAM J. STEPHENSON, Examiners.

1. A PROCESS FOR CASTING A MATERIAL SELECTED FROM THE GROUP CONSISTINGOF COPPER AND COPPER-CONTAINING ALLOYS COMPRISING PREPARING ABORAX-SILICA CASTING SLAG CONSISTING OF BETWEEN ABOUT 20 AND ABOUT 30PERCENT BY WEIGHT SILICA AND BETWEEN ABOUT 80 AND ABOUT 70 PERCENT BYWEIGHT BORAX, AND UP TO AN INSUBSTANTIAL AMOUNT OF IMPURITIES WHICH AREINERT TO THE MATERIAL TO BE CAST AND WHICH DO NOT SUBSTANTIALLY INCREASETHE FREEZING POINT OF THE BORAX-SILICA SLAG, AND SLAG HAVING A FREEZINGPOINT BELOW THE FREEZING POINT OF THE MATERIAL TO BE CAST; FORMING AMOLTEN POOL OF SAID CASTING SLAG ON THE BOTTOM OF A CASTING FORM;FORMING A SHELL OF SAID CASTING SLAG ON THE INNER WALLS OF SAID CASTINGFORM; AND POURING A MOLTEN MASS OF THE MATERIAL TO BE CAST INTO SAIDCASTING FORM AND THROUGH SAID POOL OF CASTING SLAG AT A RATE SUCH THATONLY THE INNER SURFACE OF SAID SHELL IS MELTED.